To enhance telephone service, a telephony applications processor (AP) can be attached to a private branch exchange (PBX) or a central office (CO). One such AP is voice store and forward, or voice mail. Voice mail can use voice message interfacing or enhanced voice message interfacing to determine information identifying the caller and called party.
Occasionally, there may be a need to transfer an incoming call from one AP port to another AP port. For example, assume that the AP has two ports or interfaces. One of the ports (port1) has voice capability only; the other (port2) can provide voice and facsimile (fax) service. If an incoming call initially accesses port1 but requests fax service, it should be transferred to port2. Typically, the transfer is accomplished by flashing the hookswitch on port1, placing the call on hold, initiating a transfer through the PBX with the proper code, answering the call on port2, and then disconnecting port1.
If no other call is attempting to seize port2, the transfer will be achieved. However, with multiple users accessing the AP, occasionally a different caller could seize port2 before the transfer can be made from port1, resulting in a race or glare condition. This places the first caller in an unforeseen hold situation, even though it may have received an indication that the transfer was going through.
In another situation, voice mail units may be distributed across a network. To access one's mailbox, the user must enter on a specific subscriber line or on a specific set of lines. Where there are a limited number of APs distributed over a network, it may be preferable from a management perspective to have users enter the system on a single incoming line. The call must then be transferred to the proper line in order to reach a particular AP.
It would be desirable therefore to obtain confirmation of a port's availability before a transfer is attempted, allowing the call to queue, if need be, for the desired service and have some mechanism of assuring a successful transfer once one is initiated.